Coordinated circuit protection enhances AC-LED luminaires and lamps (MAGAZINE)

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This article was published in the June 2011 issue of LEDs Magazine. View the Table of Contents and download the PDF file of the complete June 2011 issue.

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In recent years, LED technology has made impressive gains in price, performance and manufacturability, and further improvements that optimize LED operation are expected to drive exponential growth in the LED lighting market. Among the most recent commercial developments are AC-LEDs, which can operate directly on line voltage, without the need for an AC to DC converter. But even AC-LEDs are susceptible to line transients that can overheat the components, either causing immediate failure or greatly shortening the useful life of the LEDs. This article will describe a circuit-protection strategy that can help prevent overheating and deliver a reliable solid-state lighting (SSL) product.

When proper thermal management and circuit-protection design strategies are employed, AC-LED technology offers several advantages over conventional lighting, including compact size, component count reduction, energy efficiency, and reduced system cost. But lamp and luminaire designers must design to protect against over-voltage, over-current, and over-temperature conditions.

FIG. 1. As with any electronic system operating on line voltage, unprotected AC-LED lighting systems can be damaged by lightning surges that create voltage spikes or ring waves (oscillating waves generated by lighting or other switching elements in the AC power system). A metal oxide varistor (MOV) is often used to help protect lighting systems from lightning surges and ring-wave effects, and helps manufacturers meet safety and performance standards. The MOV clamps short-duration voltage impulses. Lightning tests according to IEC 61000-4-5 and ring-wave tests according to IEEE C.62.41 can be used to simulate these real-life threats in the lab.

SSL designs may need a second protection device for over-current and over-temperature conditions. In both AC-LEDs and DC-LEDs alike, excessive heat at the LED junction can dramatically reduce both the light output and lifespan of the LED.

FIG. 2. TE Circuit Protection’s PolySwitch polymeric positive temperature coefficient (PPTC) devices help provide over-current and over-temperature protection and can be easily integrated onto a circuit board with the AC-LED. The PPTC acts like a fuse to limit current in a series circuit that drives the LED, yet can automatically reset itself when the fault clears.

Thermal runaway design considerations

To understand the circuit-protection needs, let’s consider the potential problem with LEDs due to the fact that they exhibit a negative thermal coefficient. The LED forward voltage (Vf) decreases with increasing temperature. With a constant-voltage power supply, increasing the temperature slightly decreases Vf and increases the current by a relatively large amount. This causes a net increase in power and an increase in temperature. For a given thermal path, the heat transfer may no longer be able to accommodate the increasing power.

In extreme cases, the result is thermal runaway, which may damage the AC-LED if the current or power is not limited to prescribed levels. The LED light output varies roughly linearly with the current passing through the LED, within a specified current range. If the current exceeds the manufacturer’s recommendations, the LEDs can become brighter but the light output can degrade at a faster rate due to lumen depreciation, shortening the useful life of the component.

FIG. 3. Every effort should be made to keep the LED cool for maximum efficiency. LEDs are typically coupled to a heat sink to minimize the junction temperature. Limiting the current or power through an AC-LED helps prevent thermal runaway and has the added benefit of providing better brightness regulation over a wide temperature range, as well as a longer lifetime.

Input voltage (V)	70	80	90	100	110	120	130	140	150
Current (mA)	11	13	22	33	44	52	68	86	103 dropped to 13
Temperature (°C)	26	29	36	49	64	79	102	128	142
Light output (lux)	110	430	1010	1710	2320	2950	3550	3800	4000 dropped to 420

1. Light output is measured at 5-inch distance.
2. Temperature is junction temperature.
3. Test results will differ with different resistors, thermal design and ambient temperature.
TABLE 1. Test results from a Seoul Semiconductor AN3211 LED with PolySwitch miniSMDC014F protection, using the test setup shown in Fig. 2.

Let’s examine an SSL design based on the AN3211 AC-LED from Seoul Semiconductor (Fig. 5). Externally, the LED is treated as an AC-driven component that requires a combination of external resistors to set the operating current based on its forward voltage. The LED’s internal construction consists of two sets of diodes in parallel that are of opposite polarity. As shown in Fig. 1, as the line voltage increases on the positive half-cycle, one of the internal strings conducts and generates light; during the negative half-cycle, the other string conducts and generates light.

Fig. 2 shows the Seoul Semiconductor AC-LED AN3211 test setup, where an MOV and a PolySwitch miniSMDC014F PPTC device were integrated on the AC-LED lighting-system board. The MOV serves to limit damage from voltage spikes while the PPTC protects the LED from current and temperature faults.

FIG. 4. Table 1 shows the test parameters and results from a series of tests run on the sample circuit, during which the increase in voltage ultimately caused an over-current condition. The tests exhibited normal operation through an increase from 70V to 140V on the input. When the input voltage was raised to 150V the PPTC opened, making the bypass resistor the only path for current flow, and thereby reducing the current draw from 103 mA to 12 mA. The light output also dropped in a corresponding fashion, but the LED was protected. Designers can achieve different results by varying the resistors used in the circuit in Fig. 2.

Over-voltage protection design considerations

FIG. 5. Together the MOV and PPTC protect against both voltage transients and over-current faults – whether the over-current is caused by a spike from lightning or another source. Indeed even impulse changes in voltage can produce a disproportionate change in current and affect light output, so the LEDs must be protected from these AC-line-voltage fluctuations. MOVs provide the primary transient over-voltage suppression in AC-line-voltage applications and help prevent transient over-voltage damage. The PPTC device helps limit the current flowing through the LED during steady-state over-current operation and helps protect it from voltage fluctuations.

Fig. 3 and Fig. 4 show test results on the AN3211 protected with a 7-mm, 200V MOV when lightning and ring-wave tests are applied to the lighting circuit. As seen from the results, the MOV solves lightning and ring-wave issues by damping the surges. Also, the light output is very stable during lightning surge and ring-wave testing. This approach helps equipment remain operational after specified lightning tests according to IEC 61000-4-5 and ring-wave tests according to IEEEC.62.41.

AC-LEDs have been used increasingly in general-purpose Illumination systems for indoor and outdoor use. Although the AC systems offer excellent reliability and design benefits, they also require robust protection to meet safety and performance standards.

Design risk can be minimized by pairing a PPTC over-current/over-temperature protection device with an MOV to provide a completely-resettable and coordinated circuit-protection solution. New products in development by Seoul Semiconductor include over-current, over-voltage and over-temperature protection provided by TE Circuit Protection’s AC 2Pro device. This hybrid device combines a PPTC element with an MOV in one thermally-protected device, and gives designers flexibility and the convenience of integrated protection in a single device.